Refrigeration systems often include heat exchangers comprising aluminum tubing that is soldered to copper tubing used to convey a refrigerant fluid to and from the heat exchanger. The two dissimilar metals produce a galvanic couple at their union, and in the presence of moisture or other electrolyte, galvanic corrosion occurs at the copper/aluminum interface. Aluminum, being more anodic than copper, deteriorates much more rapidly at the junction of the two metals, eventually causing a leak through which refrigerant may escape to the atmosphere.
This problem has been less serious in the past because the end of the copper tube could be inserted inside the end of the aluminum heat exchanger tube to form a solderable lap joint. In such a joint the aluminum that overlapped the copper first started corroding at the end of the alumnium tube. A leakage failure at the joint did not occur until the corrosion progressed to the other end of the overlap portion of the joint. With sufficient overlap, joint failure did not occur within the expected life of the refrigeration unit. Moreover, solder built up at the end of the aluminum tube could further delay joint failure.
Recently, however, it has been found desirable to use aluminum tubing having an internally roughened surface in constructing the heat exchanger to improve its heat transfer and thus improve the efficiency of the refrigeration system. This roughened surface includes a variety of forms such as helical or fluted fins which introduce turbulence to the refrigerant flowing through the heat exchanger tubing. But the roughened surface does not provide a mating surface for joining to another tube because the grooves on that surface form capillary channels that draw the solder or braze material away from the joint. Therefore, instead of the roughened inner surface, the smooth outer surface of such internally enhanced tubes are often used as the mating surface and are soldered to the inner surface of the adjoining copper tube. Unfortunately, however, the galvanic corrosion starts at the copper/aluminum interface where the exterior of the joint is exposed to moisture. Since this interface is at the end of the copper tube, the corrosion only needs to penetrate the thickness of the aluminum tube to cause a leakage failure at the joint. Unless this corrosion is inhibited, failure can occur well within the expected life of the refrigeration unit.
Therefore it is an object of this invention to inhibit galvanic corrosion at a joint of two tubes of dissimilar metals by protecting the joint from atmospheric moisture.
Another object is to cover the joint with a protective tubing that only requires the application of heat to shrink it tightly around the joint.
Still another object is to use a protective tubing that includes a thermosetting adhesive that not only seals out moisture but also seals any small refrigerant leaks at the joint.
These and other objects of the invention will be appararent from the attached drawing and the description of the preferred embodiments which follow hereinbelow.